A typical electronic image sensor comprises a number of light sensitive picture elements (“pixels”) arranged in a two-dimensional array. Such an image sensor may be configured to produce a color image by forming an appropriate color filter array (CFA) over the pixels. Examples of image sensors of this type are disclosed in U.S. Patent Application Publication No. 2007/0024931, entitled “Image Sensor with Improved Light Sensitivity,” which is incorporated by reference herein.
As is well known, an image sensor may be implemented using complementary metal-oxide-semiconductor (CMOS) circuitry. In such an arrangement, each pixel typically comprises a photodiode and other circuitry elements that are formed in a silicon sensor layer on a silicon substrate. One or more dielectric layers are usually formed above the silicon sensor layer and may incorporate additional circuitry elements as well as multiple levels of metallization used to form interconnects. The side of the image sensor on which the dielectric layers and associated levels of metallization are formed is commonly referred to as the frontside, while the side having the silicon substrate is referred to as the backside.
In a frontside illuminated image sensor, light from a subject scene is incident on the frontside of the image sensor, and the silicon substrate is relatively thick. However, the presence of metallization level interconnects and various other features associated with the dielectric layers on the frontside of the image sensor can adversely impact the fill factor and quantum efficiency of the image sensor.
A backside illuminated image sensor addresses the fill factor and quantum efficiency issues associated with the frontside dielectric layers by thinning or removing the thick silicon substrate and arranging the image sensor such that light from a subject scene is incident on the backside of the image sensor. Thus, the incident light is no longer impacted by metallization level interconnects and other features of the dielectric layers, and fill factor and quantum efficiency are improved.
A problem that can arise in backside illuminated image sensors relates to ensuring proper alignment between image sensor features that are formed using frontside processing operations with those formed using backside processing operations. For example, photodiodes may be formed in the sensor layer using frontside processing operations while corresponding color filter elements of the above-noted CFA are formed using backside processing operations. Features formed using frontside processing operations are referred to as frontside features, while features formed using backside processing operations are referred to as backside features.
Typical conventional approaches utilize frontside alignment marks to align different lithography masks that are applied in various frontside processing operations used to form the frontside features. However, such techniques fail to adequately address the alignment of backside features with the frontside features. Thus, it remains difficult under conventional practice to align backside features such as the color filter elements of the CFA with frontside features such as the photodiodes of the sensor layer. Any alignment mismatch between the CFA elements and the photodiodes can significantly degrade the performance of the image sensor.
Accordingly, a need exists for improved processing techniques for forming backside illuminated image sensors.